In a magnetic recording device in which a read head is based on a spin valve magnetoresistance (SVMR) or a giant magnetoresistance (GMR) effect, there is a constant drive to increase recording density. One method of accomplishing this objective is to decrease the size of the sensor element in the read head that is suspended over a magnetic disk on an air bearing surface (ABS). The sensor is a critical component in which different magnetic states are detected by passing a sense current there through and monitoring a resistance change. The popular GMR configuration includes two ferromagnetic layers which are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields. In the absence of an external magnetic field, the magnetization direction of the free layer is aligned perpendicular to that of the pinned layer by the influence of abutting hard bias layers on opposite sides of the sensor stack. When an external magnetic field is applied by passing the sensor over a recording medium on the ABS, the free layer magnetic moment may rotate to a direction which is parallel to that of the pinned layer.
A sense current is used to detect a resistance value which is lower when the magnetic moments of the free layer and pinned layer are in a parallel state. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the sensor stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the cross-sectional area of the sensor is typically smaller than 0.1×0.1 microns at the ABS plane. Current recording head applications are typically based on an abutting junction configuration in which a hard bias layer is formed adjacent to each side of a free layer in a GMR spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important so that edge demagnification in the free layer must be reduced. In other words, horizontal (longitudinal) biasing is necessary so that a single domain magnetization state in the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity.
In longitudinal biasing read head design, films of high coercivity material are abutted against the edges of the GMR sensor and particularly against the sides of the free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for sensor elements become smaller with higher recording density requirements, the minimum longitudinal bias field necessary for free layer domain stabilization increases.
A high coercivity in the in-plane direction is needed in the hard bias layer to provide a stable longitudinal bias that maintains a single domain state in the free layer and thereby avoids undesirable Barkhausen noise. This condition is realized when there is a sufficient in-plane remnant magnetization (Mr) which may also be expressed as Mrt since Mr is dependent on the thickness (t) of the hard bias layer. Mrt is the component that provides the longitudinal bias flux to the free layer and must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of a reasonably sized external magnetic field. Moreover, a high squareness (S) hard bias material is desired. In other words, S = Mr/MS should approach 1 where MS represents the magnetic saturation value.
Referring to FIG. 1, a conventional read head 1 based on a GMR sensor configuration is shown and is comprised of a substrate 2 upon which a first shield layer 3 and a first gap layer 4 are formed. There is a GMR element comprised of a bottom portion 5a, a free layer 6, and a top portion 5b formed on the first gap layer 4. Note that the GMR element generally has sloped sidewalls wherein the top portion 5b is not as wide as the bottom portion 5a. The GMR element may be a bottom spin valve in which an AFM layer and pinned layer (not shown) are in the bottom portion 5a or the GMR element may be a top spin valve where the AFM layer and pinned layer are in the top portion 5b. There is a seed layer 7 formed on the first gap layer 4 and along the GMR element which ensures that the subsequently deposited hard bias layers 8 have a proper microstructure. Hard bias layers 8 form an abutting junction 12 on either side of the free layer 6. Leads 9 are provided on the hard bias layers 8 to carry current to and from the GMR element. The distance between the leads 9 defines the track width TW of the read head 1. Above the leads 9 and GMR element are successively formed a second gap layer 10 and a second shield layer 11.
The pinned layer in the GMR element is pinned in the Y direction by exchange coupling with an adjacent AFM layer that is magnetized in the Y direction by an annealing process. The hard bias layers 8 which are made of a material such as CoCrPt or CoPt are magnetized in the X direction as depicted by vectors 13 and influence an X directional alignment of the magnetic vector 14 in the free layer 6. When a magnetic field of sufficient strength is applied in the Y direction from a recording medium by moving the read head 1 over a hard disk (not shown) in the Z direction, then the magnetization in the free layer switches to the Y direction. This change in magnetic state is sensed by a voltage change due to a drop in the electrical resistance for an electrical current that is passed through the GMR element. In a CIP spin valve, this sense current IS is in a direction parallel to the planes of the sensor stack.
One concern about the output signal from a GMR element during a feed back (read) operation is that the asymmetry sigma should be as small as possible in order to accurately reproduce the waveform from the recording medium. Asymmetry is determined by the variable magnetization direction of the free layer. Ideally, the magnetic moment 14 of the free layer 6 is orthogonal to the magnetic moment of the pinned layer when no external magnetic field is present. However, the actual angle between the aforementioned magnetic moments usually deviates somewhat from 90° because of other magnetic forces in the GMR element and thereby produces an asymmetric waveform in the output.
Another concern is that CoPt or CoCrPt films which have been used as the biasing layer for magnetic recording for quite some time have a large intrinsic anisotropy. As a result, the easy axes of the hard bias layers tend to distribute randomly along the junction edge because of a tapered junction shape caused by the shadowing effect of a photoresist mask during sensor fabrication. Thus, the biasing direction will become randomized along the junction edge and thereby decrease the biasing strength and efficiency. In other words, dispersions in biasing strength are formed and must be reduced to achieve a high performance sensor element.
To improve the hard bias design, it is desirable to have a well aligned hard bias layer or to find some way to align the biasing direction of the hard bias layer, especially along the sloped wall of the junction. One effective way of aligning the hard biasing direction is to make use of exchange coupling between a magnetic underlayer and a hard bias layer. As a result, signal stability is significantly improved while maintaining high sensor sensitivity. Because an underlayer such as FeCoMo has a magnetization that is very high compared with a typical hard bias layer and its magnetic softness is not as low as desired, a new soft underlayer material and a new hard bias structure are necessary for the next generation recording head. In other words, a low softness underlayer is desirable since it has fewer easy axis dispersions which lead to improved alignment of the hard bias structure at the abutting junctions.
Other related art is found in U.S. Pat. No. 6,118,624 where a hard biasing configuration consisting of a high saturation magnetization layer such as CoPt or CoCrPt on a hard magnetic layer like Fe85Co15 is described. Unfortunately, the high magnetic saturation of FeCo alloys generally leads to undesirably high magnetostriction.
A method of suppressing dispersions in a magnetic recording device is disclosed in U.S. Pat. No. 6,661,627 wherein a solenoid is used to adjust a magnetic field running in a longitudinal bias direction. Therefore, a constant effective track width and optimum signal output can be obtained. However, the method for fabricating this device appears to be complicated and expensive.
Improved lattice matching is achieved in U.S. Pat. No. 6,577,477 by inserting a CrCoTa buffer layer between a seed layer of CrTi or Cr and a hard bias layer that is CoPt or CoCrPt. In U.S. Patent Application 2004/0105192, a thin NiCr, Ni, Fe, or Cr film is inserted between the seed layer and an antiferromagnetic (AFM) layer to screen out structural distortion resulting from the long junction tail of a bottom spin valve structure. An interlayer comprised of Cr, CrV, or CoCrTa is formed between a seed layer and hard bias layer in U.S. Pat. No. 6,185,081 to promote in-plane c-axis growth of Co in the hard bias layer.
In U.S. Pat. No. 6,807,034, a bias underlayer formed on an insulating layer in a CPP dual spin valve configuration is preferably a metal film with a body centered cubic (BCC) crystal structure in which the (110) plane is preferably oriented. Related U.S. Pat. Nos. 6,656,604 and 6,587,316 describe bias underlayers with a BCC structure and made of Cr, Ti, Mo, or WMo that separate an AFM layer from an overlying hard bias layer and enable the magnetization easy axis of the hard bias layer to be planar oriented and the hard bias magnetic field to be intensified.
A metal buffer layer formed on an AFM layer and along the sides of a free layer in an MR element is disclosed in U.S. Pat. No. 6,690,554 and increases the magnetic strength of an overlying hard bias layer.
In U.S. Pat. No. 6,667,493, bias underlayers formed of a non-magnetic material (Cr, W, Mo, V, Mn, Nb, or Ta) have sidewall portions formed along the sides of a multilayer film and base portions formed on a substrate wherein the thickness of the sidewall portions that contact the free layer is greater than that of the base portions.
A Sendust-type alloy is employed as an underlayer for a hard bias layer in U.S. Pat. No. 6,449,135.